The field of this invention is sensing systems, particularly of the type used on cranes to detect two blocking situations.
For lifting devices of all types, two blocking, as it is called, provides a very significant health and safety factor for those working about such a lifting operation. Two blocking is the lifting by the lifting line of a travelling member up into the uppermost supporting member resulting in damage to the uppermost portion of the supporting member and the sheaving mechanism therewith, the travelling block and/or the load. This two blocking situation typically occurs when the lifting line is hoisted too rapidly with the operator not paying proper attention to the proximity of the travelling member to the supporting member, such as a boom point of a boom of a crane, resulting in the travelling member colliding with the supporting member with resultant damage to both. Not only is there substantial risk that the lifting line may snap in such a situation resulting in a free falling load with the potential dangers to those below, the line may also snap and whip into proximity of the operator resulting in substantial risk of injury thereto.